Shielded cathode



June 1966 B. P. DE LANY ET AL 3,258,632

SHIELDED CATHODE Filed Oct. 14, 1963 2 Sheets-Sheet l June 28, 1966 B.P. DE LANY ET AL 3,258,632

SHIELDED CATHODE Filed Oct. 14, 1963 2 Sheets-Sheet 2 f g f 33 Maw/wiff/l ff/V United States Patent 3,258,632 SHIELDED CATHODE BeatricePearson De Lany, 36 La Gorce Circle, Miami Beach, Fla, and Paul L.Copeland, 17 W. 80 Oak Lane, Bensenville, Ill.

Filed Oct. 14, 1963, Ser. No. 315,861 11 Claims. (Cl. 313-199) Thisinvention relates to a cold cathode glow discharge gas tube and inpartcular to a tube of the type described having a compound cathodewhich substantially reduces sputter and evaporation of the cathodematerial.

A primary purpose of the invention is to provide a glow discharge tubein which the electrodes, which may be similar, are formed with a hole,and there are means within the hole providing a region of low electricfield adjacent the electron emitting surface of the electrode.

Another purpose is a compound cathode for use in a tube of the typedescribed in which there are means within the hole of the cathode forslowing the positive ions prior to impingement or bombardment on theelectron emitting surface of the cathode.

Another purpose is a tube of the type described including a generallycylindrical or tubular cathode and a screen placed within the hole ofthe cathode and maintained at a potential slightly more positive thanthat of the cathode to slow down the positive ions prior to impingementon the electron emitting surface of the cathode.

Another purpose is a compound cathode of the type described in which theplasma within the cathode hole is positioned within the screen.

Another purpose is an improved tube construction of the type describedsuitable for providing illumination and for other purposes, such asrectifiers, relays, oscillators and regulators.

Other purposes will appear in the ensuing specification, drawings andclaims.

The invention is illustrated diagrammatically in the following drawingswherein:

FIGURE 1 is a side elevation of a tube of the type described,

FIGURE 2 is an enlarged section along the axis of one of the electrodesillustrated in FIGURE 1,

FIGURE 3 is an end view of the electrode illustrated in FIGURE 2,

FIGURE 4 is a diagram on which are plotted the curves illustrating theeffect of the presence or absence of the shield,

FIGURE 5 is a section illustrating a modified form of the invention inwhich only a single cathode of the type disclosed in FIGURE 2 is used,

FIGURE 6 illustrates a further modification of the electrode arrangementin which a plurality of metal sheets is used, and

FIGURE '7 illustrates diagrammatically the zigzag path of a neutralmolecule in migrating between a grid plate and an emitter plate.

The efiiciency of ionization of a hollow cathode makes it attractive foruse in applications of glow discharge tubes, for example in lightingtubes. The current density of such cathodes is substantially higher thanfor a simple flat cathode. Heretofore, pronounced sputtering has madethe use of such a hollow cathode undesirable for lighting. Sputteringreduces the life of the cathode as it destroys any layer placed on thesurface of the 'ice cathode to reduce the work function. Furthermore,the material removed from the cathode is deposited close to the cathodeorifice where the discharge concentrates. Since the material of thecathode is of necessity an electrical conductor, and is ordinarily ametal or a semi-conductor or an alloy of a metal, this reduces theinsulation of parts close to the cathode which may seriously limit theeffective life of any control device using the hollow cathode. Also,since the deposit is opaque, the walls of the tube tend to become blackand there is a substantial reduction in illumination. The presentinvention is concerned with reducing sputtering so as to make a hollowcathode-type electrode usable in illumination tubes.

Sputtering depends upon the energy of the ions striking the cathodesurface. It has been known that sputtering is negligible for ionenergies of less than about 20 electron volts, although the exactcritical energy for sputtering depends on the kind of ion and the natureof the cathode surface. This invention is directed to providing positiveion impingement of the cathode surface of a sutficient low energy toprevent or substantially reduce sputtering.

Successful maintenance of a satisfactory glow discharge depends onpositive ions producing secondary electron emission. It has beendetermined that at low energies there is little dependence of secondaryelectron emission on the kinetic energy of the positive ions. Low speedions are effective in liberating secondary electrons, but they do notcause sputtering of the cathode. By reduc ing the speed or energy of thepositive ions that are to impinge the electron-emitting surface of thecathode, it is possible to prevent sputtering. This reduction in thekinetic energy of the positive ions can be accomplished by providing alow electric field in the space through which the positive ions move.There is low gain in energy which passes through such an electric field.

The ions are interacting by various mechanisms with the molecules of thegas through which they are moving, and, if the electric field is low,they lose more energy in these interactions than they gain from theiracceleration in the electric field and in consequence of this, they slowdown. Thus the energy of the positive ions impinging on the cathode canbe reduced to values so low that the sputtering from the cathode surfaceis negligible.

The kinetic theory of gases provides the means for checking thefeasibility of accomplishing these objectives. A gas filling the tubeconsists of a very large number of extremely small molecules in veryrapid motion. For hydrogen gas at room temperature, the effective speedof the molecules is about a mile per second. For larger molecularweights, the molecules have less average speed. For example, themolecules of argon travel less than a quarter as fast as those ofhydrogen. Pressure on the walls of the containing vessel is caused bythe momentum transfer when these molecules strike and are reflected fromthe walls.

A molecule under laboratory conditions is unlikely to travel withconstant velocity from one wall of the container to the opposite wall.Ordinarily it collides repeatedly with other molecules and has itsmomentum changed many times. The changes in velocity may be large and amolecule follows a zigzag path. The average distance between collisionsis called the mean free path. Even in gases at a relatively low pressurefor the glow discharge, namely 1.0 Torr (1.0 mm. Hg) these mean freepaths for momentum transfer are quite small. For hydrogen gas (H at 25C. the mean free path is 0.0093 cm. while other gases have mean freepaths which are comparable. A gas of perfect spheres, to have the samemean free path as hydrogen, would require that the spheres be only 2.751O- cm. in diameter. While to have the same mean free path as argonwould require ideal spheres 3.67 lO cm. in diameter. The area of acircle having a radius equal to the diameter of the ideal sphere iscalled the kinetic theory cross section of the molecule.

Individual molecules change their position by zigzag paths, and for theindividual straight line segments each direction in space is equallylikely. The distance which a molecule will travel in migrating betweentwo parallel planes is very much greater than the separation betweenthese planes. If, for example, the planes are separated by u mean freepaths, on the average the number of mean free paths which a moleculetravels in migrating from one plane to the other is 12 Theseconsiderations apply in a general way also to the ions passing throughthe shield on their way to the cathode. If there is no electric field inthis region, they are simply slowed down in their collisions with themolecules of the gas. After losing their forward momentum, they followzigzag paths similar to the molecules in the preceding discussion. Ifthere is a low electric field in the region, the general considerationsare much the same except that the positive ions between the collisionshave a small acceleration in the direction of the field. On the average,in consequence of this acceleration, there is a relatively slow drift ofthe ions in the direction of the field.

One means of providing such a low electric field adjacent theelectron-emitting surface of the cathode is to provide a shield orscreen which is at the same or a slightly different potential than thatof the cathode. Such a screen or shield establishes a potentialdifference between itself and the anode such that the plasma of thedischarge is set up within it. The field in the neighborhood of thecathode surface is reduced and there is a space through which the ionsdrift with small acceleration before they impinge on the cathodesurface. Such a shield does not substantially reduce the number ofelectrons liberated from the surface by the positive ions, but doesreduce sputtering and prolongs effective cathode and tube life.

As illustrated in FIGURE 1, a tube It may include an air imperviousenvelope 12, for example glass or the like, within which is a suitablegas under a sufiicient pressure. The noble or other inert gases, mercuryvapor, or mixtures of them are satisfactory gases. At each end of thetube there is an electrode 14 and when the tube, either clear or coatedwith phosphers, is used for lighting the electrodes will besubstantially identical. This is conventional when tubes of this typeoperate on alternating current, and each of the electrodes arealternately functioning as an anode and cathode.

FIGURE 2 illustrates the details of the electrodes 14. Each of theelectrodes may include a generally cylindrical or tubular mass 16 whichmay be formed of any material which suffices to provide strong electronemission when positive ions impinge upon it. Tungsten, tantalum,molybdenum, nickel and stainless steel may all be satisfactory. Any oneof these materials may have a coating of a substance which lowers thework function. There is a hole 18 positioned within the mass 16 andpreferably the hole is along the axis of the electrode. A practicalsize, although the invention should not be so limited, is to have themass 16 with a diameter of about one-half inch with the hole 18 having adiameter of about inch.

Positioned within the hole 18 and spaced therefrom is a fine screen orthe like 20 which may be formed of aluminium, magnesium, tantalum orsome other satisfactory metal or semi-conductor or alloy. Thesematerials may have oxide coatings when this reduces the sputtering ofmaterial. The screen 20 may extend the length of the hole, although thisis not necessary. A lead-in 22 for the screen passes through aninsulating bushing 24 at the end of the hollow cathode. A lead-in 26provides the potential for the cathode and preferably the potentialdifference between the screen and the cathode is very slight with thescreen being a small amount more positive than the cathode. A suitablebiasing resistor, not shown, may be placed between leads 22 and 26 sothat the leads may then be connected to a common source of alternatingcurrent.

When the tube is operating the plasma with its column of positive ionswill be formed within the screen 20. The positive ions thus formed inthe plasma are then moved by the potential of the cathode and the screento impinge on the interior surface 28 of the hole 18 to produce thesecondary emission for forming the glow discharge in the tube. Thescreen is to be located in the cathode dark space of the glow. Thescreen is used to reduce the electric field in the portion of thecathode dark space near the cathode surface. In this region of lowpotential gradient, there is little acceleration of the positive ions asthey travel to the cathode. Preferably the spacing between the screenand the surface 28 or the width of the sheath is such that the mean freepath of the positive ions moving across this sheath is small relative toits thickness. Most of the ions lose their kinetic energy near thecathode and the low speed ions formed cannot gain much energy beforecoming in contact with the principal electron emitting surface and thereis therefore little sputtering.

The screen or shield should not be limited to any part-icular size orthickness as what is important is to provide a particular potential atthis point. The screen should be sutficiently small however to keep itssputter at a negligible level. Any one of the materials listed may besatisfactory for forming the screen. In like manner the size andconfigurations of the cathode mass may vary widely. A cylindrical shapeis not necessary. The gas pressures and the type of gas will vary andparticular combinations of gas pressure, cathode material, and screenmaterial may be used in different applications of lighting.

The effectiveness of a shield in improving the electricalcharacteristics of a hollow cathode is an important advantage. Forexample, we have used a pair of plates, each 6 inches square, separatedfrom one another by 3 inches and operated with such gas pressures thatthe interior surfaces of the arrangement serve as sources of electronemission. For purposes of comparison, one such cathode was made withouta shield. The second parallel plate cathode was identical to the firstexcept that each plate of the pair had a shield of fine wires near thesurface facing the opposite plate. Each wire of the shield was separatedby 3 mm. from the interior surface of the nearer parallel plate and thespacing between adjacent wires was 2 mm. The entire surface of eachplate was shielded by such a wire screen. Both plates of each cathodewere connected directly to a source of negative potential. an easycomparison between the operation of the shielded and the unshieldedparallel plate was obtained by electrically connecting each shield tothe cathode plate near it. A conventional anode was made positive withrespect to the cathode by a measured amount, and the total currentpassing from each cathode pair to the anode was measured. At gaspressures of 0.1 Torr 4) or greater the plasma of the discharge wasestablished between the parallel plates. For equal potentialdifferences, the current in the shielded cathode was substantiallylarger than the current in the unshielded cathode. Representative datais plotted in FIGURE 4, and it is seen that including shields in acathode cavity results in about twice and often considerably more thantwice the current at a given potential difference between anode andcathode.

When milliammeters are connected to measure the currents in the shieldsand parallel plates separately and the potentials are made substantiallythe same by connecting the milliammeters to the negative terminal of thepower source, the current supplied by the shields is only slightly lessthan that supplied by the two plates. As the potential of the shields israised, the shield current decreases gradually until it reaches zerowhen the potential of the shields is about 200 volts positive withrespect to the parallel plates. This represents the floating potentialof the shields. Any desired potential between zero and the floatingpotential for the operation of the shields may be established simply byvarying a resistor placed in series with the electrical connection tothe shields.

When the word sputtering is used in the specification and claims, it isto be understood as meaning the loss of cathode material resulting frompositive ion bombardment. It is to be understood that the shield inwhatever form it is present does not discriminate against secondaryemission of electrons but does reduce sputtering.

As used herein, the expression a material of low work function is to beunderstood as meaning a material for which the increase in potentialenergy of an electron, as it passes from the interior of the material toa matterfree space, is relatively small.

In particular, the electrode may take the form illustrated in FIGURE 5,in which, although there are two electrodes, only one is of the typeshown generally in FIGURE 1 and in detail in FIGURES 2 and 3. There isthus in the form of FIGURE 5 an envelope 30 which is of insulatingmaterial and except for its shape it is the same as the envelope 12within which is a suitable gas under suflicient pressure, generally asdescribed in detail in connection with the description of FIGURE 1.Because it is identical with the electrode 14 shown in the earlierfigures, the electrode which is in the smaller end of the envelope 30carries the same reference numerals as those shown in FIGURES 1, 2 and 3and it need not be described here. The electrical connections are alsothe same and preferably will include a resistance 26 as shown in FIGURE2. In FIGURE 5, instead of two electrodes 14 as shown in FIGURE 1, asimple plate anode 31 is provided. It is connected to a lead in wire 32which passes through the envelope 30. The structure of FIGURE 5comprises a half wave rectifier.

FIGURE 6 illustrates a further modified form of cathode. There is shownin this rfig-ure a hollow cathode which instead of being of rounded orcircular cross section as shown in FIGURE 3 comprises a plurality ofparallel metal sheets 33, 3-3. Adjacent each is a shield 34.

The shields 34 act as does the shield or screen 20 of FIGURES 2 and 3and will normally be made of the same material as the shield 20 whichhas been described in detail above. In the form of FIGURE 6, the metalsheets are connected electrically or at the same potential and theelectrode formed of the two plates 33 shown particularly in the form ofFIGURE 6 is positioned within an envelope of suitable size and shape andis provided with a conductor the same as or equivalent to the conductor26. Electrodes according to the form of FIGURE 6 may be used in the samemanner as the electrode '14 is used, in pairs as shown (in FIGURE 1, orsingly with a conventional anode as shown in FIGURE 5.

The diagram of FIGURE 7 illustrates in detail a zigzag path of a neutralmolecule in migrating between the plane of the shield 34 and the planeof the emitter plate 33. A positive ion in the fiel-d free space wouldhave a path very similar in appearance to that shown in FIG- UR E 7. Themolecule path shown in FIGURE \'7 is typical of the migration of aneutral molecule under the conditions prevailing in the tube. It is notnecessarily however a precise showing of the movements of any particularmolecule.

Whereas the preferred form of the invention has been shown and describedherein, it should be realized that there are many modifications,substitutions and alterations thereto within the scope of the followingclaims.

We claim:

1. In a glow discharge gas tube, an air impervious envelope, a firstelectrode in said envelope effective to operate as an anode, a secondelectrode spaced from said vfirst electrode and effective to operate asa cathode, said second electrode being generally cylindrical in form andhaving a generally cylindrical hole along its axis, a generallycylindrical screen positioned within said hole and uniformly spaced fromthe interior surface of said second electrode, said generallycylindrical screen being operated at a potential slightly different thanthat of said second electrode and being effective to produce a region oflow electric field adjacent the interior surface of said secondelectrode from which electrons are extracted by positive ionimpingement.

2. In a glow discharge gas tube, an air impervious envelope, a firstelectrode in said envelope effective to operate as an anode, a secondelectrode spaced from said first electrode and effective to operate as acathode, said second electrode being generally cylindrical in form andhaving a generally cylindrical hole along its axis, a generallycylindrical screen positioned within said hole and uniformly spaced fromthe interior surface of said second electrode, said generallycylindrical screen being operated at a potential approximately the sameas that of said second electrode and being effective to produce a regionof low electric field adjacent the interior surface of said secondelectrode from which electrons are extracted by positive ionimpingement.

3. In a glow discharge tube, an air impervious envelope, a firstelectrode in said envelope effective to toperate as an anode, a secondelectrode spaced from said first electrode and effective to operate as acathode, said second electrode including -a principal electron emittingsurface and an open electrically conducting structure spaced from saidemitting surface a distance equal to several times the mean free path ofpositive ions moving thereby, said open structure being supported by thepart containing said principal electron emitting surface and maintainedat an electrical potential with respect to said emitting surface whichis effective to reduce the electric held between said electricallyconducting structure and said emitting surface.

4. The structure of claim 3 further characterized in that said secondelectrode is formed of a material of high electrical conductivity andincludes a cavity, said electrically conducting structure beingpositioned within said cavity.

5. The structure of claim 4 further characterized in that saidelectrically conducting structure includes a conducting sheet withperforations.

'6. The structure of claim 5 further characterized in that saidconducting sheet with perforations is formed of a screen.

7. The structure of claim 3 further characterized in that said secondelectrode is formed of a material of high electrical conductivity andhas an electron emitting surface coated with a film of material selectedfor its high secondary emission under impingement of positive ions.

8. In a glow discharge tube, an air impervious envelope, a firstelectrode in said envelope effective to operate as an anode, a secondelectrode spaced from said first electrode and effective to operate as acathode, said second electrode having a hole, and means within said holefor producing a region of low electric field adjacent the interiorsurface of said electrode from which electrons are extracted byimpingement of positive ions, said means being effective to reduce theelectric held over 7 a distance, from any point on the interior electronproducing surface, larger than the average mean free path of thepositive ions.

9. The structure of claim -8 further characterized in that the means forproducing a region of low electric field provides a shield of lowelectric field adjacent the electron emitting surface and of a thicknessseveral times the average mean free path of the positive ions.

10. The structure of claim 8 further characterized in that the plasma ofthe discharge is positioned within said means.

11. The structure of claim 8 further characterized in References Citedby the Examiner UNITED STATES PATENTS 5/ 1950 Menzel. 8/1960 Holmes313-209 GEORGE N. WEST-BY, Primary Examiner.

S. D. SCHLOSSER, Assistant Examiner.

1. IN A GLOW DISCHARGE GAS TUBE, AN AIR IMPERVIOUS ENVELOPE, A FIRSTELECTRODE IN SAID ENVELOPE EFFECTIVE TO OPERATE AS AN ANODE, A SECONDELECTRODE SPACED FROM SAID FIRST ELECTRODE AND EFFECTIVE TO OPERATE AT ACATHODE, SAID SECOND ELECTRODE BEING GENERALLY CYLINDRICAL IN FORM ANDHAVING A GENERALLY CYLINDRICAL HOLE ALONG ITS AXIS, A GENERALLYCYLINDRICAL SCREEN POSITIONED WITHIN SAID HOLE AND UNIFORMLY SPACED FROMTHE INTERIOR SURFACE OF SAID SECCOND ELECTRODE, SAID GENERALLYCYLINDRICAL SCREEN BEING OPERATED AT A POTENTIAL SLIGHTLY DIFFERENT THANTHAT OF SAID SECOND ELECTRODE AND BEING EFFECTIVE TO PRODUCE A REGION OFLOW ELECTRIC FIELD ADJACENT THE INTERIOR SURFACE OF SAID SECONDELECTRODE FROM WHICH ELECTRONS ARE EXTRACTED BY POSITIVE IONIMPINGEMENT.